greatly improved UX for handling links to content hosted on other instances: you shouldn’t have to use the inconspicuous search function to access it via your instance,
community collections: aggregating communities by topic each with a clear overview, their own feed and a nice, convenient way to create and view crossposts between them,
more polished and stable app(s),
ease of migrating between instances (massive bonus if we can have portable identities),
a change in how we present the core idea behind the federation model: it’s not about aggregation (this misconception leads to frustration over “fragmentation”), it’s about community self-governance/autonomy and error-correction (as in making it easier for communities to migrate if authority is abused).
Some things I think are needed first: